The present invention relates to a method and apparatus for controlling the fuel gas of a gas turbine and, more particularly, to a fuel gas controlling method and apparatus for controlling the fuel supply to a two-stage combustion type gas turbine combustor through only a primary fuel nozzle during a low-load run and through not only the primary fuel nozzle but also a secondary fuel nozzle during a high-load run.
A so-called two-stage combustion type gas turbine combustor is well known, wherein a primary fuel nozzle is arranged upstream of the combustor, whereas, a secondary fuel nozzle is arranged downstream of the combustor so that the gas turbine may be run only by the primary fuel nozzle during a low-load run at a low fuel gas flow rate and by supplying the fuel gas to the secondary fuel nozzle, too, during a high-load run. An example of such a combustor is described in "Development of a Dry Low NOx Combustor for a 120 MW Gas Turbine," ASME Paper No. 84-GT-44.
In a two-stage combustion system of the prior art, a fuel gas has a pressure thereof controlled to a constant level by a fuel gas pressure-regulating valve and then has a flow rate thereof controlled in accordance with a load control signal by a fuel control valve until the fuel gas is introduced into primary fuel nozzles of a plurality of combustors. On the other hand, the fuel gas having passed through a fuel line, branched from the downstream of the fuel control valve and through a fuel switching valve provided on the branched fuel line, is introduced through a gas manifold into a secondary fuel nozzle.
The fuel switching valve is responsive to a load signal of the gas turbine and is set to be shifted upon reaching predetermined load, from its fully closed position to its fully open position for a short period of time in response to a switching signal from a switch control unit.
In this system, the fuel gas flow rate is always exclusively controlled by the load control signal. With a low turbine load, the turbine is exclusively operated with the primary fuel of the primary fuel nozzle. For the time period required for a fuel line switching operation, the fuel supply is switched so that the fuel gas is supplied to the combustor in parallel from the primary and secondary fuel nozzles. Namely, for short period of time of the fuel switching operation, the turbine is operated in parallel by the primary and secondary fuel nozzles, and after that, the turbine is operated in parallel by the primary and secondary fuel nozzles according to a load schedule.
In this system, the operation is exclusively carried out by the primary fuel in the low-load range and, during and after the operation under a predetermined load, the two-stage combustion is required for reducing the emission of NOx so that the combustion is shifted to the two-stage combustion by additionally injecting the fuel from the secondary fuel nozzle. As mentioned above, during this switching operation, the load is controlled to maintain a constant level, and the total resultant fuel gas amount must be held at a constant level. In this state, the load control signal is held for the switching period, and the switching valve is opened. However, when the fuel gas is supplied to the secondary fuel line, the combustion gas may flow back from the combustor to the secondary fuel line to cause a backfiring. The switching valve has to be instantly shifted from its closed position to an open position to ignite a secondary fuel. At this instant, the orifice area of the nozzle is switched to flow at a flow rate from primary fuel to the sum of primary fuel and secondary fuel so that the flow rate of the fuel gas through the line increases to instantly drop the pressure at the outlet of the fuel control valve. Since the fuel control valve is under the control of the load signal, the load is instantly augmented if the increase in the gas flow rate abruptly occurs in the state of the constant load. Then, the fuel control valve operates in a closing direction thereof so as to maintain a constant load. However, due to the delay of the load signal, the total amount of the fuel gas supplied is temporarily increased to cause a load fluctuation. This is because the temperature of the combustion gas is instantly increased so that the temperature increase is sensed as the increase in the gas turbine output, i.e., as the load control signal. By these transient phenomena, the large load fluctuations, that is, the combustion gas temperature fluctuations are caused in the existing system during the switching operation, although they are very undesired for controlling the operation of the gas turbine and for the service time of the hot parts. In case the aforementioned fluctuations are remarkable, moreover, the hot parts may possibly be broken, and a highly responsive control method is accordingly required.